1. Field of the Invention
The present invention relates generally to a display system, such as a liquid crystal display system. The present invention also relates to a system for providing electrical driving of an electrode of a display system. More particularly, the invention relates to a system for electrically driving an electrode of a display system to various voltages in a controlled phase relationship to the update of pixel data.
2. Background of the Related Art
A class of display systems operate by electrically addressing a thin, intervening layer of electro-optic material, such as liquid crystal, which is positioned between two substrates. In these display systems, it is important to achieve good display characteristics including: color purity, high contrast, high brightness, and a fast response.
High independence of frames or subframes ensures the lack of coupling between intensity values at a given pixel from one frame to the next. For example, if a pixel is to be at its brightest gray level during a first frame and then at its darkest gray level at the next frame, then a high independence would ensure that this is possible whereas a low independence would cause the pixel to appear brighter than the darkest gray level during the second frame. This coupling can cause problems such as motion smearing. High frame-to-frame independence is important whether or not the display is a color or black-and-white (monochrome) or gray-scale display.
The level of contrast achievable is determined by the range of intensity attainable between the brightest gray level and the darkest gray level for a given pixel within a given frame or subframe.
In addition to contrast, it is desirable that the display be capable of displaying a bright image since brighter images are perceived as having a higher quality by a user.
Finally, the speed of display is determined by its ability to display one frame after the other at a high rate. If visual motion is to be displayed, flicker and other problems can be avoided only if the full color frames are displayed at a rate of at least 30 Hz and preferably 60 Hz or faster.
This speed requirement becomes even more stringent if the display does not contain a red, green, and blue pixel all at one pixel location (in other words, red, green and blue subpixels for each pixel location) but instead only has a single pixel. One type of such a display is a color sequential liquid crystal display as discussed in "Color-Sequential Crystalline-Silicon LCLV based Projector for Consumer HDTV" by Sayyah, Forber, and Efrom in SID digest (1995) pages 520-523. In those types of displays, if a display requires the sequential display of the red, green, and blue subframes, those subframes must be displayed at yet a rate higher than 90 HZ and preferable greater than 180 HZ to avoid flicker. For color sequential displays, high frame or subframe independence is required to display images with good color purity.
Any of the general display systems that operate by electrically addressing a thin, intervening layer of electro-optic material, such as liquid crystal, which is positioned between two substrates include the following characteristics. At least one of the two substrates is transparent or translucent to light and one of the substrates includes a plurality of pixel electrodes. Each pixel electrode corresponds to one pixel (or one subpixel) of the display, and each of the former may be driven independently to certain voltages so as to control the intervening electro-optic layer in such a way as to cause an image to be displayed on the electro-optic layer of the display. Sometimes each pixel can include a color triad of pixel electrodes. The second substrate of such a prior art display system has a single electrode, known as the common electrode or cover glass electrode, which serves to provide a reference voltage so that the pixel electrodes can develop an electric field across the intervening layer of electro-optic material.
One example of such a system is a color thin film transistor (TFT) liquid crystal display. These displays are used in many notebook-sized portable computers. Colors are generated in these displays by using RGB pixel triads in which each pixel of the triad controls the amount of light passing through its corresponding red, green, or blue color filter. These color filters are one of the most costly components of a TFT display.
The major obstacle of display systems of this type is that the results of replicating the pixel electrodes, data wire, and thin film transistors, three times at each color pixel are increased cost and reduced light transmission, requiring more peripheral backlights and increased power consumption.
The other issues of high frame-to-frame independence, high contrast, and brightness become even more difficult to achieve as display rates increase.
Many approaches have been implemented to improve display characteristics of the above type displays. One common approach involves the use of a common electrode driving circuit and driving that common electrode with as flat a common electrode rectangular driving voltage as possible. By doing so, the voltage across the liquid crystal portion at that pixel is more constant, which in turn should yield improved contrast and pixel brightness.
For example, U.S. Pat. No. 5,537,129 discloses a display system with a common electrode which attempts to achieve a flat rectangular common electrode driving voltage. Referring to FIG. 2 of that patent, a common electrode 24 is connected to its driving circuit 20 through a resistor 3b. This corrects for resistive losses at 3a and capacitive coupling to the common electrode 24 from pixels and data wires. This ensures that detection device 21 with a high input impedance can be used to make a correction so the output voltage appears to be more rectangular-like. FIGS. 5, 9b, 11(c), and 11(d) of that U.S. patent all show the desired rectangular waveforms.
Another example of this is shown with U.S. Pat. No. 5,561,442, which shows that with the properly applied common electrode voltage Vc(t) when coordinated with the previous gate wire voltage Vs(t) and the current gate wire voltage Vg(t), can yield a flat rectangular voltage V(t)-Vc(t) across the liquid crystal (C.sub.LC). This scheme involves a complicated modulation scheme coordinating modulation voltages at gate wires in relation to the modulation of the voltage at the common electrode in order to achieve their desired flat rectangular modulation of voltage across the liquid crystal.
Another approach to improving display systems which use an electro-optic material, such as a liquid crystal, is to use a miniature display with a reflective substrate. The image generated by the reflective substrate is magnified for either direct viewing or projected viewing. An advantage of this type of miniature display relative to large screen TFT liquid crystal displays is that the yield achieved in manufacturing these miniature displays is significantly higher than the yield achieved when manufacturing large screen TFT or other types of large screen liquid crystal displays. One drawback with miniature displays, however, is that the small size often places constraints on the optics used to magnify the image. For example, if it is desired to build a miniature display in a miniature housing, the optical paths between optical elements is relatively short, and this causes a number of optical problems. These problems increase when polarized light is required to be used with liquid crystal displays as is often required. The requirement of polarized light often requires a polarizing beamsplitter to be placed into the optical system. It is often difficult to obtain enough working distance between the last element in the lens of the miniature display system and the display on the reflective substrate in order to include a beamsplitting element.
The polarizers also tend to significantly reduce the effective amount of light from the illuminator because the polarizers tend to transmit only a portion of the light generated by the illuminator.
One approach to avoid these problems with miniature displays is to use polymer dispersed liquid crystal materials as the electro-optic layer in the miniature display. If polymer dispersed liquid crystals (PDLC) are used, then polarized light is not required and thus the efficiency of the use of light from the illuminator is greatly increased. Moreover, these systems do not require a beamsplitter and thus do not place as much constraints on the design of an optical system in a small head mounted miniature display system which uses reflective substrates.
A polymer dispersed liquid crystal comprises a polymer matrix which contains small inclusions of liquid crystal material. There is no global alignment present without the application of an electric field, so the director alignments of the individual inclusions of liquid crystal material are randomly oriented with respect to one another. This is shown in FIG. 19A which shows the polymer 1905 containing five inclusions 1907a, 1907b, 1907c, 1907d, and 1907e, each of which contain liquid crystal material 1909. No electric field is present between the common electrode (e.g. cover glass electrode) 1903 and the pixel electrodes 1911a, 1911b, and 1911c on the substrate. It is noted that within each inclusion of liquid crystal material, the liquid crystal material itself may be aligned within the inclusion, although this is not shown in FIG. 19A. The random alignment of the individual inclusions of liquid crystal material relative to each other causes a scattering of incident light. Thus light directed through the display or at least through the electro-optic layer is scattered.
The alignment of the liquid crystal material in all of the inclusions can be controlled by the application of an electric field. This is shown in FIG. 19B in which an electric field of +X volts is applied between the common electrode 1903 and the pixel electrodes 1911a, 1911b, and 1911c. If all the inclusion directors are made to align parallel to each other and, for example, parallel to the direction of incident light, a suitable choice of refractive index of the liquid crystal material and the polymer can cause the scattering to be substantially reduced. A modulator can thus be made by using an optical system which is sensitive to scatter. Unlike many other liquid crystal effects, the scattering effect does not require the incident light to be polarized. These types of liquid crystal displays which utilize scattering are often useful in certain display applications, especially projection display systems, because the polarizers are a major source of light loss. The scattering mechanism also lends itself to systems which use imaging optics after the liquid crystal display. These optics typically contain a mechanism which is sensitive to the scattering effects in the PDLC layer. This may be an aperture or obstruction, for example.
While miniature or larger display systems may use PDLC material for an electro-optic layer, these materials have the drawback of traditionally being relatively slow in response time and also requiring high voltages in order to achieve a change in optical state in the electro-optic layer.